Harley gearbox oil IS mineral-based. Even if you change it to synthetic 85W-140, like I did, still safe to mix mineral based 80W or whatever Manual sez. There is nothing "special" about Harley gearbox oil. Get correct weight and UR Ok.
Only haveta be careful with primary oil, wrong types can cause clutch slip.
Smart Harley owner, IQ 140, 4 years college 2 AS degrees plus working on third and A & P certificates plus avionics. Own $200K house outright, NO mortgage.
Possibly foolish if I ride any 'sickle at all, yes, but Worth it....